title: Multiple activities on phosphorylase kinase. 2. Different specificities toward the protein substrates phosphorylase b, troponin, and phosphorylase kinase.
abstract: Phosphorylase kinase exhibits three kinds of enzymatic activities. A partial activity, A0, catalyzes the phosphorylation of phosphorylase b, troponin I, and phosphorylase kinase itself (autophosphorylation); A1 can utilize only phosphorylase b and phosphorylase kinase as the substrate, whereas A2 can utilize only phosphorylase b and troponin T. Stimulation of A1 by Ca2+ coincides with an increase in the number of sites that can undergo self-phosphorylation ranging from ca. 35 to ca. 70 mol of phosphate incorporated/1.28 X 10(6) g of proteins. Inhibition of A0 and A1 by millimolar Ca2+ is accompanied by a decrease in substrate availability during self-phosphorylation. NH4Cl (150 mM) strongly inhibits the availability of troponin as a substrate. In the course of self-phosphorylation, the activities A0 and A1 are both stimulated moderately by an increase in pH; however, only A1 shows some inhibition by 150 mM NH4Cl. Millimolar Ca2+ inhibits A1 and A2 as measured by self-phosphorylation or troponin phosphorylation, as observed with the phosphorylation of phosphorylase b [Kilimann, M. W., & Heilmeyer, L. M. G., Jr. (1982) Biochemistry (preceding paper in this issue)]. The rate of self-phosphorylation varies as a function of substrate concentration (Km = 68 nM at 10 mM Mg2+ and 184 microM Ca2+, pH 9.0). The data indicate that both Ca2+ activation and inhibition seem to be mediated by phosphorylase kinase itself rather than by the substrates.
mesh_terms: D000643:Ammonium Chloride / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009124:Muscle Proteins / Q000378:metabolism*; D010764:Phosphorylase Kinase / Q000378:metabolism*; D010763:Phosphorylase b / Q000378:metabolism*; D006005:Phosphorylases / Q000378:metabolism*; D010766:Phosphorylation; D013379:Substrate Specificity; D013997:Time Factors; D014336:Troponin / Q000378:metabolism*
chemical_list: D009124:Muscle Proteins; D014336:Troponin; D000643:Ammonium Chloride; D010763:Phosphorylase b; D006005:Phosphorylases; D010764:Phosphorylase Kinase; D002118:Calcium